The present invention relates generally to a means for regulating the high pressure component of a transcritical vapor compression system.
Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. xe2x80x9cNaturalxe2x80x9d refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Unfortunately, there are problems with the use of many of these fluids as well. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide as a refrigerant to run transcritical under most conditions.
When a vapor compression system is run transcritical, it is advantageous to regulate the high pressure component of the system. By regulating the high pressure of the system, the capacity and/or efficiency of the system can be controlled and optimized. Increasing the high pressure of the system (gas cooler pressure) lowers the specific enthalpy entering the evaporator and increases capacity. However, more energy is expended because the compressor must work harder. It is advantageous to find the optimal high pressure of the system, which changes as operating conditions change. By regulating the high pressure component of the system, the optimal high pressure can be selected. Hence, there is a need in the art for a means for regulating the high pressure component of a transcritical vapor compression system.
The present invention relates to a means for regulating the high pressure component of a transcritical vapor compression system.
A vapor compression system consists of a compressor, a gas cooler, an expansion device, and an evaporator. Economizer cycles are sometimes employed to increase the efficiency and/or capacity of the system. Economizer cycles operate by expanding the refrigerant leaving the heat rejecting heat exchanger to an intermediate pressure and separating the refrigerant flow into two streams. One stream is sent to the heat absorbing heat exchanger, and the other is sent to cool the flow between two compression stages. In one form of an economizer cycle, a flash tank is used to perform the separation. This invention regulates the high pressure component of the vapor compression system (pressure in the gas cooler) by controlling the amount of charge in the flash tank. In a preferred embodiment of the invention, carbon dioxide is used as the refrigerant.
In a flash tank, refrigerant discharged from the gas cooler passes through a first expansion device, and its pressure is reduced. The refrigerant collects in the flash tank as part liquid and part vapor. The vapor refrigerant is used to cool refrigerant exhaust as it exits a first compression device, and the liquid refrigerant is further expanded by a second expansion device before entering the evaporator.
Expansion valves positioned on the path leading into and out of the flash tank are used to expand the refrigerant from high pressure to low pressure. This invention controls the actuation of the expansion valves to control the flow of charge into and out of the flash tank, regulating the amount of charge stored in the flash tank. By regulating the amount of charge stored in the flash tank, the amount of charge in the gas cooler and the high pressure of the system can be controlled.
An optimal pressure of the system can be selected by controlling the actuation of the valves. If the pressure in the gas cooler is too low, the expansion valves can be adjusted to release charge from the flash tank into the system to increase the gas cooler pressure, increasing the capacity of the system. If the pressure in the gas cooler is too high, the expansion valves can be adjusted to store charge in the flash tank to decrease the gas cooler pressure, reducing the energy expended by the compressor.
Accordingly, the present invention provides a method and system for regulating the high pressure component of a transcritical vapor compression system.
These and other features of the present invention will be best understood from the following specification and drawings.